1. Field of the Invention
This invention relates to an imaging optical system which images a line image extending in a main scanning direction in a two-dimensional image as an erected image at a unit magnification in the main scanning direction.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been put into practice as CR (Computed Radiography) a radiation image recording/reproducing system in which a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) is exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet by causing the stimulating light beam to scan the stimulable phosphor sheet in a main scanning direction while conveying the stimulable phosphor sheet in a sub-scanning direction perpendicular to the main scanning direction, the stimulated emission emitted from the stimulable phosphor sheet pixel by pixel is photoelectrically detected, thereby obtaining an image signal (a radiation image signal), and then a radiation image of the object is reproduced as a visible image on the basis of the radiation image signal on a recording medium such as a photographic film or a display such as a fine CRT.
In the radiation image read-out apparatus employed in the radiation image recording/reproducing system, in order to shorten the reading time and reduce the overall size of the apparatus and the cost, it has been proposed to use a line light source, an erecting unit imaging lens and a line sensor to photoelectrically read out the stimulated emission and obtain an image signal. For example, a linear stimulating light beam extending in a main scanning direction is projected onto the stimulable phosphor sheet running in a sub-scanning direction, and the stimulated emission emitted from the linear part of the stimulable phosphor sheet exposed to the linear stimulating light beam is imaged on a line sensor comprising a plurality of photoelectric convertor elements arranged in the main scanning direction by an erecting unit imaging lens as an erected image at a unit magnification. See, for instance, Japanese Unexamined Patent Publication No. 1(1989)-101540.
As the erecting unit imaging lens, there has been employed a refractive index profile type lens array comprising a plurality of tubular lenses (Selfoc Lens™), each having a refractive index profile in a radial direction, arranged in the main scanning direction. (U.S. Pat. No. 6,429,977)
However, such a refractive index profile type lens array comprising a plurality of cylindrical lenses cannot be sufficiently large in refractive index profile (the difference between the maximum refractive index and the minimum refractive index) in the radial direction and is difficult to increase the aperture, which results in a limited NA (numerical aperture) and unsatisfactory light collecting efficiency.
Even if the refractive index profile can be increased, there arises a problem that the resolution in the main scanning direction in which the image is formed in an erected state at a unit magnification deteriorates when the NA is enlarged to preferentially increase the light collecting efficiency since the refractive index profile increases at the same rate in both the main scanning direction and the sub-scanning direction.